(A) Field of the Invention
The present invention is related to an over-current protection apparatus, and more specifically to an over-current protection apparatus of surface mount technology (SMT) type, which is in the form of a string or an array.
(B) Description of the Related Art
To prevent over-current or over-temperature from occurring in a circuitry, the current portable electronic appliances such as cellular phones, laptop computers, portable video cameras and personal digital assistants (PDAs) commonly are embedded with over-current protection apparatuses to prevent permanent damages to the internal circuitries owing to electrical surge. Therefore, an over-current protection apparatus is serially connected to the output or input end of a circuitry in need of protection, so as to prevent core circuit or battery from being damaged.
Most electronic devices generate heat during operation, inducing the temperatures thereof to continuously increase as time goes on. Moreover, some components may generate large transient current due to interference. If an over-current protection apparatus is not employed to respond to the increasing temperature, and the large current is not cut off by breaking down circuit and increasing resistance, the relevant core circuit or battery will change the nature owing to high temperature, resulting in permanent damage.
A usual over-current protection apparatus comprises a current-sensitive element which is essentially composed of conductive positive temperature coefficient (PTC) material, and thus is a PTC device. The resistance of a PTC device is sensitive to temperature variation, and can be kept extremely low at normal operation due to its low sensitivity to temperature variation so that the circuit can operate normally. However, if an over-current or an over-temperature event occurs, the resistance will immediately increase to a high resistance state (e.g., above 104 ohm.) Therefore, the over-current will be reversely eliminated and the objective to protect a core circuit or a battery can be achieved. The PTC device may be made from polymer, i.e., the so-called polymeric positive temperature coefficient (PPTC.) Owing to the function of current-sensitivity and the nature of polymer, the over-current protection apparatus has both over-current and over-temperature protective capabilities.
A known over-current protection apparatus 10 is shown in FIG. 1. It comprises a current-sensitive layer 11, two inner electrode layers 12, two insulating layers 13, two solder-mask layers 14 and two outer electrode layers 15. The two inner electrode layers 12 are respectively disposed on the upper and lower surfaces of the current-sensitive layer 11 to form a current-sensitive element 16 of a sandwich-like structure. The current-sensitive layer 11 comprises conductive PPTC material, and thus the current-sensitive element 16 is a PPTC element also. The insulating layers 13 are respectively disposed on the upper and lower surfaces of the current-sensitive element 16 for insulation and heat conduction. The solder-mask layers 14 and outer electrode layers 15 are disposed on the surfaces of the insulating layers 13, and serve as solder-masks and bonding pads for being soldered to a printed circuit board. The sidewalls of the over-current protection apparatus 10 are provided with semi-cylindrical conductive holes 17 so as to electrically connect the upper and lower outer electrode layers 15 and the upper and lower inner electrode layers 12, where the conductive holes 17 can be made by electroplating or spreading conductive paste for electrical conduction.
The known over-current protection apparatus 10 is a single device. However, multiple devices can be installed to a printed circuit board as requested. The single device can be soldered to the printed circuit board by dipping or SMT. As usual, mass production and high density installation are employed by SMT.
Other ordinary types of over-current protection apparatuses are disclosed in U.S. Pat. Nos. 6,377,467, 5,852,397 and 6,023,403. As mentioned above, more over-current protection apparatuses are needed to protect electronic products with higher and higher integration. However, too many discrete single devices waste space of a system.